Not applicable.
REFERENCE TO MICROFICHE APPENDIX/SEQUENCE LISTING/TABLE/COMPUTER PROGRAM LISTING APPENDIX (submitted on a compact disc and an incorporation-by-reference of the material on the compact disc)
Not applicable.
1. Field of the Invention
The present invention relates to a system and method for expanding a laser beam.
2. Background Art
In many applications, the laser beam emitted by a laser needs to be expanded. Expansion of a laser beam is necessary in microlithography because a desired illumination system field is typically much bigger than the cross-sectional area of a beam emitted directly from the laser (also called a laser emission footprint). For example, one type of excimer laser beam has a laser emission footprint that is a generally rectangular area of about 5 mmxc3x9715 mm. On the other hand, an illumination system in a lithography tool may require an illumination field which is much larger on the order of 10 mm by 120 mm. This requires significant expansion of the laser beam.
One conventional approach for laser beam expansion involves the use of refractive elements, such as, a lens or prism structure. Unfortunately, expansion with such refractive elements increases the spatial coherence cell sizes of the laser and creates a speckle problem in the final image. To deal with the increasing spatial coherence cell sizes and bandwidth of the laser, a structure consisting of multiple lenses of different length has been used to expand the laser beam. Such a structure, however, is expensive and impractical for expanding a laser beam of reduced bandwidth to a degree involving hundreds or thousands of spatial cells.
Therefore, what is needed is an improved system and method for expanding a laser beam of reduced bandwidth.
The present invention relates to a method and system for expanding a laser beam without increasing the spatially coherent cell sizes. In one embodiment, an illumination system includes a horizontal reflective multiplexer and a vertical reflective multiplexer. The horizontal reflective multiplexer receives an input laser beam. The horizontal reflective multiplexer replicates the input beam along a first dimension to form a first multiplexed beam. The first multiplexed beam is made up of a series of patches representing multiplexed copies of the input beam along the first dimension. The first multiplexed beam has a first expanded footprint which covers an area greater than the input footprint. The vertical reflective multiplexer replicates the first multiplexed beam along a second dimension to form a second multiplexed beam. The second multiplexed beam is made up of copies of the first multiplexed beam. The second multiplexed beam has a second expanded footprint that has an area even greater than the first expanded footprint.
According to a further feature of the present invention, the illumination system further includes an optical subsystem that re-images cell regions of the second multiplexed beam to overlap and form an output beam. The output beam has an output footprint covering the illumination field of the illumination system.
In one embodiment, the horizontal reflective multiplexer includes a first beam splitter, second beam splitter, and mirror. The vertical reflective multiplexer includes a beam splitter and mirror. The optical subsystem for re-imaging cell regions can include, but is not limited to, a micro-lens array or a diffractive optical element.
According to a further embodiment of the present invention a method for expanding a laser beam is provided. The method includes two replicating steps. The first replicating step replicates an input beam along a first dimension to form a first multiplexed beam having a first expanded footprint. The second replicating step replicates the first multiplexed beam along a second dimension to form a second multiplexed beam having a second expanded footprint. The method can also include re-imaging cell regions of the second multiplexed beam to overlap and form an output beam. The output has an output footprint covering an illumination field.
Further features and advantages of the present invention as well as the structure and operation of various embodiments of the present invention are described in detail below with reference to the accompanying drawings.